Can a collection be modified by another thread during ToList()

Question

class Sample
{
    private List<int> _list;
    public List<int> List
    {
        get 
        { 
            return _list.Select(p => p + p).Where(q => q % 2 == 0).ToList(); 
        }
    }

    public void Add(int n)
    {
        _list.Add(n);
    }

    public void Remove(int n)
    {
        _list.Remove(n);
    }
}

I have a situation similar to above in a multithreaded environment where Add, Remove and List are accessed by multiple threads simultaneously.

In some cases Collection was modified; enumeration operation may not execute is thrown in the List getter, which makes sense as threads can Add/Remove while the Select/Where operations are executing.

Can the list be modified by another thread during a call to ToList()? Meaning, would changing the getter to

return _list.ToList().Select(p => p + p).Where(q => q % 2 == 0);

be enough, or do I need to resort to locks around access to _list

Would you consider using [concurrent collection](https://msdn.microsoft.com/en-us/library/system.collections.concurrent(v=vs.110).aspx) ? — Michael, Apr 14 '15 at 17:34

Jon Skeet · Accepted Answer · 2015-04-14T17:37:24.340

9

There's no multi-threading protection in List<T>, so yes, another thread could modify the list during a ToList call. The results could be:

Everything's fine, and the resulting list has the new value
Everything's fine, and the resulting list doesn't have the new value
It sets your hard disk on fire (or other undefined behaviour)

Basically, it is not safe to use a List<T> from multiple threads without external synchronization, unless all the threads are just reading from it. You either need synchronization (e.g. using SynchronizedCollection<T> instead of List<T>, or handling the synchronization yourself), or a concurrent collection such as ConcurrentBag<T>.

edited Apr 14 '15 at 17:37

answered Apr 14 '15 at 17:34

Jon Skeet

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2

Actually after letting a test run for a while, it threw `Destination array was not long enough. Check destIndex and length, and the array's lower bounds.` which is close enough to setting my hard disk on fire for me. Thanks Jon – Bort Apr 14 '15 at 17:39

Can a collection be modified by another thread during ToList()

1 Answers1